In electrophotographic applications such as xerography, a charge retentive surface is electrostatically charged and exposed to a light pattern of an original image to be reproduced to selectively discharge the surface in accordance therewith. The resulting pattern of charged and discharged areas on that surface form an electrostatic charge pattern (an electrostatic latent image) conforming to the original image. The latent image is developed by contacting it with a finely divided electrostatically attractable powder or powder suspension referred to as "toner". Toner is held on the image areas by the electrostatic charge on the surface. Thus, a toner image is produced in conformity with a light image of the original being reproduced. The toner image may then be transferred to a substrate (e.g., paper), and the image affixed thereto to form a permanent record of the image to be reproduced. Subsequent to development, excess toner left on the charge retentive surface is cleaned from the surface. The process is well known and useful for light lens copying from an original and printing applications from electronically generated or stored originals, where a charged surface may be imagewise discharged in a variety of ways. Ion projection devices where a charge is imagewise deposited on a charge retentive substrate operate similarly. In a slightly different arrangement, toner may be transferred to an intermediate surface, prior to retransfer to a final substrate.
Transfer of toner from the charge retentive surface to the final substrate is commonly accomplished electrostatically. A developed toner image is held on the charge retentive surface with electrostatic and mechanical forces. A substrate (such as a copy sheet) is brought into intimate contact with the surface, sandwiching the toner thereinbetween. An electrostatic transfer charging device, such as a corotron, applies a charge to the back side of the sheet, to attract the toner image to the sheet.
Unfortunately, the interface between the sheet and the charge retentive surface is not always optimal. Particularly with non-flat sheets, such as sheets that have already passed through a fixing operation such as heat and/or pressure fusing, or perforated sheets, or sheets that are brought into imperfect contact with the charge retentive surface, the contact between the sheet and the charge retentive surface may be non-uniform, characterized by gaps where contact has failed. There is a tendency for toner not to transfer across these gaps. A copy quality defect referred to as transfer deletion results.
The problem of transfer deletion has been unsatisfactorily addressed by mechanical devices that force the sheet into the required intimate and complete contact with the charge retentive surface. Blade arrangements that sweep over the back side of the sheet have been proposed, but tend to collect toner if the blade is not cammed away from the charge retentive surface during the interdocument period, or frequently cleaned. Biased roll transfer devices have been proposed, where the electrostatic transfer charging device is a biased roll member that maintains contact with the sheet and charge retentive surface. Again, however, the roll must be cleaned. Both arrangements can add cost, and mechanical complexity.
That acoustic agitation or vibration of a surface can enhance toner release therefrom is known. U.S. Pat. No. 4,111,546 to Maret proposes enhancing cleaning by applying high frequency vibratory energy to an imaging surface with a vibratory member, coupled to an imaging surface at the cleaning station to obtain toner release. The vibratory member described is a horn arrangement excited with a piezoelectric transducer (Piezoelectric element) at a frequency in the range of about 20 kilohertz. U.S. Pat. No. 4,684,242 to Schultz describes a cleaning apparatus that provides a magnetically permeable cleaning fluid held within a cleaning chamber, wherein an ultrasonic horn driven by piezoelectric transducer element is coupled to the backside of the imaging surface to vibrate the fluid within the chamber for enhanced cleaning. U.S. Pat. No. 4,007,982 to Stange provides a cleaning blade with an edge vibrated at a frequency to substantially reduce the frictional resistance between the blade edge and the imaging surface, preferably at ultrasonic frequencies. U.S. Pat. No. 4,121,947 to Hemphill provides an arrangement which vibrates a photoreceptor to dislodge toner particles by entraining the photoreceptor about a roller, while rotating the roller about an eccentric axis. Xerox Disclosure Journal "Floating Diaphragm Vacuum Shoe, by Hull et al., Vol. 2, No. 6, Nov./Dec. 1977 shows a vacuum cleaning shoe wherein a diaphragm is oscillated in the ultrasonic range. U.S. Pat. No. 3,653,758 to Trimmer et al., suggests that transfer of toner from an imaging surface to a substrate in a non contacting transfer electrostatic printing device may be enhanced by applying vibratory energy to the backside of an imaging surface at the transfer station. U.S. Pat. No. 4,546,722 to Toda et al., U.S. Pat. No. 4,794,878 to Connors et al., and U.S. Pat. No. 4,833,503 to Snelling disclose use of a piezoelectric transducer driving a resonator for the enhancement of development within a developer housing. Japanese Published Patent Appl. No. 62-195685 suggests that imagewise transfer of photoconductive toner, discharged in imagewise fashion, from a toner retaining surface to a substrate in a printing device may be enhanced by applying vibratory energy to the backside of the toner retaining surface. U.S. Pat. No. 3,854,974 to Sato et al. discloses vibration simultaneous with transfer across pressure engaged surfaces. However, this patent does not address the problem of deletions in association with corotron transfer.
Resonators for applying vibrational energy to some other member are known, for example in U.S. Pat. No. 4,363,992 to Holze, Jr. which shows a horn for a resonator, coupled with a piezoelectric transducer device supplying vibrational energy, and provided with slots partially through the horn for improving non uniform response along the tip of the horn. U.S. Pat. No. 3,113,225 to Kleesattel et al. describes an arrangement wherein an ultrasonic resonator is used for a variety of purposes, including aiding in coating paper, glossing or compacting paper and as friction free guides. U.S. Pat. No. 3,733,238 to Long et al. shows an ultrasonic welding device with a stepped horn. U.S. Pat. No. 3,713,987 to Low shows ultrasonic agitation of a surface, and subsequent vacuum removal of released matter.
Coupling of vibrational energy to a surface has been considered in Defensive Publication T893,001 by Fisler which shows an ultrasonic energy creating device is arranged in association with a cleaning arrangement in a xerographic device, and is coupled to the imaging surface via a bead of liquid through which the imaging surface is moved. U.S. Pat. No. 3,635,762 to Ott et al. and U.S. Pat. No. 3,422,479 to Jeffee show a similar arrangement where a web of photographic material is moved through a pool of solvent liquid in which an ultrasonic energy producing device is provided. U.S. Pat. No. 4,483,034 to Ensminger shows cleaning of a xerographic drum by submersion into a pool of liquid provided with an ultrasonic energy producing device. U.S. Pat. No. 3,190,793 Starke shows a method of cleaning paper making machine felts by directing ultrasonic energy through a cleaning liquid in which the felts are immersed.
It has been noted that even with fully segmented horns, as shown in copending application assigned to the same assignee as the present application, and entitled, "Segmented Resonator Structure having a Uniform Response for Electrophotographic Imaging" by inventors W. Nowak et al. and assigned Ser. No. 7/548,517, there is a fall-off in response of the resonator at the outer edges of the device. A similar fall off is shown in U.S. Pat. No. 4,363,992 to Holze, Jr., at FIG. 2, showing the response of the resonator of FIG. 1.
All the references cited herein are specifically incorporated by reference for their teachings.